1. Field of the Invention
This invention relates to a technology to fabricate a crystalline silicon film, and relates to a semiconductor device including the crystalline silicon film obtained with the application of this invention, and its manufacture technology.
2. Related Art
A semiconductor device using a crystalline silicon film for a semiconductor layer including at least a channel formation region, a source region, and a drain region formed thereon, and typically an active matrix type display device wherein a circuit is formed by combining a Thin Film Transistor: TFT to be built in and used as a drive circuit, has been widely used.
In a crystalline silicon film, a high field effect mobility (high-speed movement of a carrier) is obtained as compared with a field effect mobility of TFT (0.5 to 1 cm2/Vs) which uses the conventional amorphous silicon film. For this reason, the crystalline silicon film is considered as not only suitable for a display device which is further required for high definition, but also suitable for integration of a drive circuit. Accordingly, technical development for obtaining a crystalline silicon film with good crystallizability is further pursued.
A liquid crystal display device is mentioned as typical equipment using TFT which uses a crystalline silicon film as a circuit element. The market of the liquid crystal display device is expanding rapidly, making use of the advantages such as a low power consumption and space saving, to find the way into a display part of electrical appliances such as a liquid crystal television or a view finder of a video camera and an optical system of an LCD projector.
In recent years, the LCD projector is becoming widely used in ordinary homes also, and responding to user's demand for a high luminance, intense light such as a xenon arc lamp for example is used as a light source. However, when the light from this intense light source carried out incidence onto a semiconductor layer, a carrier was induced to flow in silicon, to thereby raise a problem of leakage current being generated by light.
In case of a silicon film with good crystallizability such as a grain boundary, especially a continuous grain boundary (with small energy barriers of the grain boundary part and few crystal defects in a grain), lattice matching of the crystal grain boundary is good and carrier life is long. Therefore, the lifetime of an electron/hole pairs excited by light also tends to serve as leakage current (also referred to as light leakage current) of TFT as it is. The light leakage current, causing a contrast fall, needs to be reduced.
Therefore, as a measure against the light leakage current, technical development to form a light shielding film on TFT is generally pursued for interrupting the light before carrying out incidence onto the semiconductor layer. However, light intensity of a lamp increased every year responding to the demand for a high luminance. Consequently, even stray light generated by diffraction or abundant reflection inside a semiconductor device is becoming the intensity of not disregarded as an optical leakage factor. The light shielding film needs to be formed in order to shield even this stray light, involving the problem of increase in large manufacture cost.
In view of the above problem, object of this invention is to achieve a semiconductor device capable of reducing the light leakage current of TFT and performing a good display by suppressing the optical sensitivity of the semiconductor film itself.